1. Field of the Invention
The invention relates to a continuous dense phase conveying method and apparatus for conveying large quantities of dry bulk materials through a material conveying pipe.
2. Description of the Prior Art
Pneumatic conveying systems are designed to move large quantities of particulate material in an air, or inert gas, medium through a material conveying pipe. The use of the term "particulate material" herein includes any material which is in a granular, powder, or pelletized form including, but not limited to, pellets formed of plastic material or other chemicals, bulk commodity grains, soap powders, and other similar bulk materials, as are known in the art. Pneumatic conveying systems are of two general types: dilute phase systems and dense phase systems. Typically, the type of particulate material to be conveyed determines what type of pneumatic conveying system to utilize.
Dilute phase conveying systems typically utilize a high volume of air, or inert gas, at a low pressure, not exceeding 15 psig, that is used to convey a low volume of particulate material. The air, or inert gas, stream travels at a high velocity, typically between 4,000 to 6500 feet per minute to keep the light particulate product load in suspension in the flowing air, or inert gas, stream. In dilute phase conveying systems, the particulate material being conveyed is usually very free flowing. A major disadvantage associated with dilute phase conveying systems is that if the particulate material being conveyed is either abrasive, or is subject to damage by the high air, or inert gas, stream velocity, dilute phase conveying systems should not be used. If the particulate material is abrasive, it can damage the material conveying pipe and related equipment, and thus cause premature failure of such equipment. With many particulate materials, product degradation caused by the high velocity air, or inert gas, stream is a significant problem, because a significant portion of the particulate material to be conveyed may be damaged while it is being conveyed. With many particulate materials, such as, for example, plastic pellets, an undesired amount of unwanted fines, or a fine powder, results from the conveyance of such materials through a dilute phase conveying system.
If the particulate material to be conveyed is either abrasive or subject to product degradation, dense phase conveying systems have been previously utilized. Dense phase conveying systems are characterized by utilizing lower air, or inert gas, velocities and a much higher conveying pressure. The conveying pressure may be as high as 60 psig, and the air, or inert gas, streams travel at velocity typically not exceeding 1200 feet per minute. In contrast to dilute phase conveying systems, dense phase conveying systems utilize higher ratios of particulate material to the amount of air, or inert gas, used. Instead of keeping the particulate material in suspension in the air, or inert gas, stream, the particulate material is pushed, or extruded, through the material conveying pipe in dunes, balls, or "slugs". Since the air, or inert gas, velocity is much lower, the solid particulate material is no longer suspended in the air stream, but falls in the form of strands to the bottom of the material conveying pipe where they are moved through the material conveying pipe by the force exerted upon them by the flowing air, or inert gas, stream, as is well known in the art. When enough particulate material is fed into the material conveying pipe, the dunes and balls formed are moved ahead until the dunes or balls hit each other and form solid plugs, or slugs of particulate material. Since the air, or inert gas, cannot pass through the formed plug, the gas pressure behind the plug pushes, or extrudes, the plug down the material conveying pipe while the next plug, or slug, is being formed. The slugs will be of varying length, depending upon the nature of the particulate product being conveyed, and will typically follow each other along the material conveying pipe with a space in between successive slugs of particulate material.
Because of the high pressures necessary for dense phase conveying systems, the use of ASME code pressure containment vessels, or pressure pots, is required which are typically first filled with the particulate material to be conveyed, as by gravity feeding of the particulate material into the pressure pot. The pressure containment vessel, or pressure pot, must then be sealed, pressurized to the desired high pressure, and after the desired high pressure is achieved, the bottom of the pressure containment vessel would be opened to allow the particulate material to flow out and be forced down the material conveying pipe to its intended destination. The pressure containment vessel would then have to be closed and the pressurized air vented off before the pressure containment vessel can be recharged for the next batch of particulate material to be conveyed. Alternatively, the pressure containment vessel could be returned to the necessary lower pressure for refilling by letting the high pressure air, or inert gas, dissipate down the material conveying line. The foregoing dense phase conveying system is typically referred to as a batch dense phase conveying system.
A disadvantage associated with batch dense phase conveying systems is that there is a significant amount of non-product conveying time involved while the pressure containment vessel is venting, being refilled, and pressurized. Another disadvantage is that, because of the significant amount of non-conveying time, the conveying pipe size must be relatively large to compensate for the lost time. Another disadvantage is that the last portions of the particulate material leaving the pressure containment vessel tend to accelerate rapidly as the material conveying pipe is blown free and the high pressure gas seeks to dissipate down along the material conveying pipe. The increased acceleration of the particulate material being conveyed often causes a significant amount of product degradation to the last portion of the particulate material being conveyed. A further disadvantage results from the high cost of the necessary pressure containment vessel.
Multiple pressure containment vessels have been utilized for continuous dense phase conveying systems, wherein two pressure containment vessels, or pressure pots, are connected to the material conveying pipe. While one vessel is being filled, the other vessel is being discharged into the material conveying pipe. Although this type of continuous dense phase conveying system is somewhat of an improvement over batch dense phase conveying systems, it still suffers from the disadvantages of increased complexity, greater costs, and greater space requirements for the equipment. Additionally, when the desired amount of particulate material has been conveyed, there is still a significant amount of product degradation to the last portion of the particulate material being conveyed, due to the increased velocity of the air, or inert gas, at the end of the last cycle. Another disadvantage is that it is difficult to purge the material conveying pipe clean when two pressure containment vessels are used.
Another prior art dense phase conveying system has utilized a high pressure rotary valve to feed the particulate material into the material conveying pipe, whereby the dense phase conveying system could be operated in a continuous mode that resembles a dilute phase conveying system. A disadvantage associated with this prior art continuous dense phase conveying system is that when conveying is started, the initial high air pressure and high volume of the system causes the particulate material in the material conveying pipe to be conveyed at an undesired high velocity, because of a lack of enough particulate material in the material conveying pipe to form the desired slugs of particulate material in the system. Undesired product degradation results from such high velocity conveyance of the particulate material. Another disadvantage associated with this prior art continuous dense phase conveying system is that, upon reaching proper operating conditions for dense phase conveying, erratic, or other undesired, feeding of the particulate material into the material conveying pipe can cause the conveying system to revert back to the undesired high velocity air, or gas, conveying stream, until desired operating conditions are again reached. An additional disadvantage of such prior art dense phase conveying system is that when the desired quantity of particulate material has been conveyed, the last portion of the particulate material is conveyed at an undesired high velocity and subject to material degradation therefrom. This occurs because after the feeding of the particulate material has ceased, the high pressure air, or inert gas, still flows into the material conveying pipe. A final disadvantage associated with such prior art continuous dense phase conveying system is that if too much particulate material is fed into the material conveying pipe, the material conveying pipe can be completely plugged by the particulate material and the air, or inert gas, pressure within the material conveying pipe rises to an undesired level, whereby it can become necessary to shut down the entire system until the proper pressure conditions can be maintained within the material conveying pipe.
Accordingly, prior to the development of the present invention, there has been no continuous dense phase conveying system and method for conveying a particulate material which: is simple and efficient to manufacture and use; is economical to manufacture and use; does not require an excessive amount of space for the necessary equipment; prevents degradation of the particulate material being conveyed at the time product conveying begins and ends; prevents undesired over-plugging of the material conveying pipe and resulting undesired pressure increases; and prevents degradation of the particulate material being conveyed upon erratic feeding or insufficient feeding of particulate material into the material conveying pipe. Therefore, the art has sought a continuous dense phase conveying system and method for conveying particulate material which: is simple and efficient to manufacture and use; is economical to manufacture and use; does not require extensive space for the necessary equipment; prevents degradation of the particulate material being conveyed at the beginning and end of the conveying cycle; prevents undesired over-plugging of the material conveying pipe and undesired pressure increases within the material conveying pipe; and prevents degradation of the particulate material during erratic feeding or insufficient feeding of the particulate material into the material conveying pipe.